The Spartan-3A Family of devices can be configured via the BPI interface and can then pass data to a downstream serial chain.
How should this be implemented in the tools?
To implement the Serial Daisy Chain from a Spartan-3A device when loading from a BPI flash, you will need to set the following BitGen option for the S-3A devices handling the BPI to serial output conversion.
This BitGen setting can be added by right-clicking the "Generate Programing File" process tab in ISE, selecting properties, and adding an additional bitgen command line option.
This option will enable the setting in the device required to pass data out on the DOUT pin at a CCLK rate 8 times faster than the BPI interface.
There is also an option in iMPACT when generating a PROM file that will specify the Downstream Daisy-Chain Type, and this will need to be set to Serial.